Many firearms, such as assault rifles that are commonly used in military and law enforcement situations are designed by their manufacturer to be gas-operated. The AR-15 family of firearms, including the M16-type firearms, illustrates examples of assault rifles that are designed by their manufacturer to be gas-operated. M16-type firearms are a military version of the AR-15 family of firearms and are capable of operating in a fully automatic mode. M16-type firearms have been manufactured by companies including, but not limited to, Colt Manufacturing Company, the ArmaLite Division of Fairchild Aircraft and Engine Company, BushMaster Firearms Incorporated and Fabrique Nationale.
As originally designed, the AR-15, M16 and M4 firearms are collectively and generically referred to in the industry as “M16-type” weapons. M16-type weapons are auto loading and are usually either semi-automatic, full-automatic, burst-fire, selective-fire, or a combination of the above. As such, M16-type weapons are subjected to higher volumes of firing than many sporting type firearms and they are accordingly subjected to higher levels of heat, fouling and component failures.
M16-type weapons have been the primary service weapons of the US Armed Forces and many of its allies for more than forty years. M16-type weapons are usually “gas-operated” as disclosed by Stoner in U.S. Pat. No. 2,951,424. Generally, the Stoner gas-operated system has a gas block that is mounted to the barrel and a bolt carrier that is designed to reciprocate in an upper receiver. The barrel has a gas-regulating orifice known as a “gas-port” residing under the gas block. The purpose of the gas-port is to control the amount of gas delivered to the gas system from the bore, the size of which has an effect on the cyclic rate of the weapon. The bolt carrier acts as a gas cylinder and possesses a “key” mounted to its upper surface. The bolt acts as a piston and is housed within the bolt carrier. A gas tube is connected to the gas block on one end, and passes through a hole in the upper receiver to interface with the bolt carrier key, through a telescoping arrangement between the two parts. The purpose of the gas tube is to communicate gas pressure from the gas block on the barrel, to the bolt carrier and bolt arrangement. In most cases, the gas tube has an offset step on one end versus the other; i.e. the two ends do not share a common bore axis and the hole in the gas block for receiving the gas tube is not in alignment with the hole in the upper receiver for receiving the opposite end of the gas tube. Upon firing a cartridge, the projectile in the barrel passes a gas port in the barrel, and some of the hot expanding gasses that are propelling the projectile escape into the gas port. These expanding gasses and resulting pressure are in turn transmitted to the bolt carrier (cylinder) and bolt (piston) arrangement. Upon entering the cavity in the bolt carrier, the pressure forces a separation of the bolt and carrier and propels them in opposite directions. As the bolt is fixed against the breech face, the bolt carrier travels in the recoil direction. Some residual gas is vented through ports in the bolt carrier to the outside of the upper receiver. When the telescoping key leaves the end of the gas tube, remaining gasses in the gas tube are vented into the upper receiver body where the bolt carrier travels and consequently into the internal operating components of the weapon. Upon reaching full recoil, an action spring returns the bolt carrier in the counter-recoil direction toward its forward static position.
M16-type weapons have a known history of problems associated with gas-operation of its bolt carrier group. Some of these problems were disclosed in the 1967 Ichord Investigation, which sought answers to M16-type weapon failures in the combat zones in which US soldiers were fighting. In other reports, it has been revealed that the gas system of the M16 rifle is sensitive to certain ammunition propellants containing calcium carbonate as an ingredient. Carbon deposits and resulting fouling from the M16-type gas system can compromise the reliability of the weapon. This fouling makes it difficult for an operator to clean his M16 -type weapon, particularly in a combat zone where opportunity to clean weapons may be difficult to encounter. Other component failures have been attributed to the heat delivered by the gas tube to the M16-type weapon's internal components, such as ejector and extractor springs. Gas tube failures occur when the weapon is subjected to high levels of firing in a short amount of time, generating excessive heat and thereby causing the thin gas tube to enter a plastic state whereby it “droops” or bursts. Still more problems and failures have been attributed to the M16 -type weapon's high cyclic rate, especially in the shortened carbine variants which attempt to unlock the breech bolt under higher pressures.
In the aftermath of the Ichord Report, Colt's Patent Firearms designed the “Model 703” rifle. The Model 703 used a long-stroke, gas-piston device similar to an AK-47 that required an entirely new upper receiver assembly be fitted to M16-type lower receivers, which is an expensive proposition. While there exists little information on this system, it can be surmised from publicly available photographs that it likely suffered from increased weight, bolt carrier binding in the receiver extension tube, and manufacturing and logistical difficulties due to the number of components that were unique to this system.
There have been prior attempts to modify M16-type rifles for gas-piston operation such as the Taiwanese T65 rifle, which is a factory built weapon that utilized a piston design similar to that disclosed in the Miller U.S. Pat. No. 3,246,567. The T65 lacked the familiar bolt closure device common to most modern M16-type weapons as disclosed in Sturtevant U.S. Pat. No. 3,326,155 and was short-lived for various reasons, including unacceptable receiver wear caused by an overturning moment of the bolt carrier and maintenance difficulty.
The commercially marketed “Rhino” device disclosed in Langendorfer U.S. Pat. No. 4,244,273 sought to modify M16-type weapons by re-working the gas block to house the piston assembly. The bolt carrier key was then reinforced with a shear pin and an impact buffer was added to it in an attempt to absorb the impact loads transmitted by the operating rod. This device suffered from the same excessive receiver wear as the T65, and its “bolt on” key would shear off of the carrier when repeatedly impacted by the operating rod. Additionally, a weapon utilizing the “Rhino” device had to be removed from service and permanently converted to accept the new components, i.e., the “Rhino” device was not a “drop-in” conversion assembly. Thus, this device was deemed frail and unsuitable for military service and it failed in the marketplace.
Many other weapons are referred to as “piston-operated”, which are generally either “short-stroke” or “long-stroke” pistons. Some examples of short-stroke, piston-operated weapons are the FN-FAL, Armalite AR-18, Taiwanese T65, Russian SKS, Ultimax 100 and Steyer AUG. Some examples of long-stroke, piston-operated weapons are the German STG-44, Russian AK-47, Stoner 63, Beretta AR-70 and Robinson M96. Generally, a short-stroke, piston-operated weapon has an operating rod that is separate from the bolt carrier. A piston is energized upon firing and propels the operating rod into contact with a portion of the bolt carrier to bias it in the recoil direction. The travel distance of a short-stroke piston and operating rod are usually a fraction of the overall travel distance of the bolt carrier. Generally, a long-stroke, piston-operated weapon has a piston and operating rod that is engaged with the bolt carrier. A piston is energized upon firing and propels the operating rod and bolt carrier in the recoil direction. The travel distance of a long-stroke piston and operating rod are usually equal to the overall travel distance of the bolt carrier.
Shortened carbine variants of M16-type weapons have a shorter barrel and a shorter gas tube. Accordingly, the gas-port is located nearer the chamber compared to the rifle-length variants of the M16-type weapons. Because of the gas-regulating port location in such shortened carbine variants, the gas-port is subjected to premature erosion by heat and unburned powder particles. This port erosion causes an additional increase in cyclic rate of fire by permitting higher gas levels to act upon the bolt and bolt carrier. Higher cyclic rates cause impact energy to the moving parts to increase by the square of the velocity. This can result in a higher incidence of component failures such as bolts, bolt carriers, extractors, extractor pins, cam pins and barrel extensions.
It is important for weapons used in combat and law enforcement situations to be reliable and easy to clean and maintain in the field. For example, it is desirable that M16-type combat weapons display higher safety levels of the critical components so that they will not fail in combat, and injuries to troops and/or law enforcement personnel caused by catastrophic component failures should be reduced as much as possible. It is also desirable that M16-type combat weapons be “upgraded” by unit armorers without the need for removing the weapons from military or police inventories or from combat zones.
It is desirable that solutions to inherent problems of the M16-type weapon be economical to implement. It is also desirable for such solutions to utilize as many of the host weapon's original components as possible, thereby increasing production and resulting in a minimal impact on the logistical support network of military and law enforcement communities. It is further desirable that the upgraded weapons be familiar to the personnel using them so that training costs and time are kept to a minimum.
Therefore, an approach for retrofitting an M16-type weapon in a manner that overcomes known deficiencies associated with gas-operation of its bolt carrier group would be advantageous, desirable and useful.